A musical system such as a system for recording, playing, amplifying a string instrument, comprises a string instrument equipped with microphones and an amplifier and finally loudspeakers for sound generation.
The amplifier in such a system will affect the perceived sound of the listener/musician due to non-linearity of the amplifier used in the reproduction of the signal from the microphone. In the earliest known amplifier technology radio tubes were used. These early amplifiers based on radio tubes are perceived by many listeners as superior to modern amplifiers based on semiconductor technology. The main cause of audible differences between tube based amplifiers and previously known semiconductor amplifiers are the differences in the non-linearity involved.
Tube based amplifiers in general have high supply voltages and output transformers for transforming the impedance to the loudspeaker. Radio tubes are also relatively expensive and are mechanically fragile; also the availability of good radio tubes becomes more and more limited. The necessary output transformers are also relatively expensive and heavy.
Therefore, it is desirable to mimic the sound shaping properties of the radio tube by using modern semiconductor components. This has been done in several earlier attempts which can be divided into two main groups of amplifiers.
The first type of amplifiers uses standard discrete components that utilize some nonlinearity effect of semiconductors. As an example of this can GB 2274367A be pointed out, which uses high voltage MOSFET (Metal Oxide Semiconductor Field Effect Transistors) transistors. Also U.S. Pat. No. 4,405,832A discloses a sound shaping stage that aims to mimic the distorted sound of a tube amplifier by means of operational amplifiers and diodes.
In the first group belong also amplifiers disclosed on the internet in forums for guitar enthusiasts. Among these the so called “Fetzer Valve” (see http://www.runoffgroove.com/fetzervalve.com) can be found. This amplifier is equipped with a copy of the input stage of a Fender tube amplifier, replacing the radio tubes with JFET transistors. The idea behind this amplifier is based on the understanding that a JFET transistor and a radio tube exhibit similar type of non-linearity. However, there are several disadvantages with the “Fetzer Valve” design; the most serious disadvantage from a production point of view is that the amplifier must be manually adjusted with potentiometers for obtaining desired properties. Another disadvantage is that the potential of the gate terminal is adjusted with an auto-bias design. The auto-bias causes the design to become sensitive for intrinsic variations of the field effect transistor.
Another amplifier is the “Tillman”-preamplifier published in the Usenet discussion forum alt.guitar on the Sep. 28, 1992. The “Tillman” amplifier also uses a JFET transistor and an auto-bias of the potential of the gate terminal.
The other group of amplifier uses digital sound processing to mimic the tube sound. The digital sound processing is performed by digitalization of the input signal from the microphones of the guitar; the digital signal is then fed to a digital signal processor, which processes the digital signal according to a program. The program is designed to emulate the non-linearity that the tube amplifier exhibits. As an example of this can the U.S. Pat. No. 5,789,689 be pointed out.